Resistor-diode attenuator



June 13, 1967 A. FRISCH ETAL RES I STOR -DIODE ATTENUATOR Filed May 13,1964 United States Patent inesne assignments, to Tektronix, Inc.,Beaverton, 0reg., a corporation of Oregon Filed May 13, 1964, Ser. No.367,153

3 Claims. Cl. 333-81 This invention relates to power adjustment means ofthe type often referred to as attenuators, and more particularly to anattenuator for selectively adjusting the amount of power supplied to aload from a source of alternating current power.

One of the most common potentiometer.

A potentiometer is a resistor having an adjustable tap, i.e., a resistorhaving two ends, and a fixed value of resistance as measured betweenthese ends, and a third terminal whose position is adjustable to anypoint between these ends. If a source of power is connected across thefixed terminals and a load is placed between either fixed terminal andthe tap, then the potentiometer may be said to act as an attenuator.This is because the voltage, or alternately, the power, available at theload is smaller (or at most equal to) that supplied by the source.

Theoretically a potentiometer is a pure resistance. However, inactuality the various electrical connections involved in apotentiometer, including the elements of the sliding terminal, offer acertain amount of capacitive and inductive impedance. Such impedance canbe ignored when the potentiometer is being employed with a source ofdirect current or with an alternating current source of relatively lowfrequency. However, since the effect of these impedances increases withthe frequency of the voltage applied to the potentiometer, when apotentiometerattenuator is'employed with a high frequency source ofpower, e.g., a radio frequency source, the effects of these impedancesbecome considerable and cannot be ignored. As a consequence, theusefulness of potentiometer-attenutaors is limited to installationsinvolving relatively low frequencies. Another disadvantage of apotentiometer, as an attenuator in A.C. installations, is the fact thatthe impedance which it presents does not remain constant, but rathervaries with the frequency of the source and the setting of theattenuator. This leads to a further disadvantage, which is that theattenuation produced by a potentiometer varies with the frequency of thesource.

Types of attenuators other than potentiometers are, of course, Wellknown. There are purely mechanical attenuators,' but these tend to bebulky and expensive. In addition, there are electrically controlledattenuators, but these may not be continuously variable, may not exhibitconstant impedance, and have limited bandwidth.

It is an object of the present invention to provide an attenuator, theinput impedance of which does not vary with the frequency of the powersource, or with the attenuation setting.

It is another object of the invention to provide an attenuator whereinthe attenuation produced at any particular setting is subsantiallyindependent of frequency.

It is a further object of the invention to provide such an attenuatorhaving no moving parts, the degree of attenuation produced by theattenuator being electrically controlled.

It is still another object of the invention to provide an types ofattenuators is a attenuator which is compact, inexpensive tomanufacture, and thoroughly practical throughout a large range offrequencies.

To achieve these objects, the present invention provides an attenuatorcomprising plural resistors series connected between a source ofalternating current power and a load, and plural variable resistancemeans connected to the junctions between the resistors.

When current flowing through one of the series-connected resistors fromthe source reaches a junction, it divides; a portion of the currentflows toward the variable resistance means and the remainder flowstoward the load. Obviously, the amount of current which flows toward thevariable resistance means depends upon the selected resistance of themeans. A feature of the invention is the employment of an electricallycontrolled variable resistance means, such as a diode, the resistancevalue of which depends upon the value of DC. voltage supplied to it.Means are provided for permitting continuous variation of the DC.voltage supplied to the variable resistance means in order to vary theamount of power supplied to the load.

An extremely important feature of the invention is the provision ofresistors in parallel with the input and output sides of the attenuatorfor reducing the input and output voltage standing wave ratios of theattenuator and making them as close to unity as possible.

Other objects and advantages of the invention will be apparent from thefollowing description in which reference is made to the accompanyingdrawing.

The drawing is a schematic diagram of an attenuator constructed inaccordance with the present invention.

The attenuator is shown connected between two terminals 10 and 11. Theterminal 10 is connected to a source of alternating current power, andthe terminal 11 is connected to a load to be supplied by the source viathe attenuator. The source and load are not shown.

The attenuator chosen to illustrate the present invention employs threediodes 12, 13, and 14 which serve as variable resistance means. However,it is to be understood that any number of diodes may be used. The diodes12-14 are connected, respectively, to three junctions 15, 16, and 17,the junctions being connected in series with one another and with theterminals 10 and 11. The diodes which are arranged to pass current inthe direction of the junctions, are connected in parallel with oneanother and with the load connected to terminal 11. Biasing voltage issupplied to the diodes from a DC. source 20. The anodes of all thediodes 12-14 are connected to a common junction 21, and arranged inseries between the DC. source 20 and the junction 21 is a potentiometer22 capable of continuously varying the value of biasing voltage suppliedto the diodes.

The endmost diodes 12 and 14 are isolated from the center diode 13 byresistors 23 and 24, respectively, the former being located between thejunctions 15 and 16, and the latter being located between the junctions16 and 17. Furthermore, the diode 12 is isolated from the sourceterminal 10 by a resistor 25 connected between terminal 10 and junction15, and the diode 14 is isolated from the load terminal 11 by a resistor26 connected between terminal 11 and junction 17. The four isolatingresistors 2326 are therefore connected in series between the terminals10 and 11, and are preferably equal to one another in value. Connectedto a junction 29, between resistor 25 and terminal 10, in parallel withresistor 25 is a padding resistor 30, and connected to a junction 31,between resistor 26 and terminal 11, in parallel with resistor 26 isanother padding resistor 32. The purpose of the padding resistor is toreduce the input and output voltage standing wave ratios (VSWR) of theattenuator and bring these ratios as close to unity as possible. Thisfunction will be described in more detail below.

Connected in series between the junction 21 and the anode of each of theendmost diodes 12 and 14 is a balancing resistor 33. The resistor 33insures that the current flowing to the endmost diodes from the DCsource 20 equals the current flowing to the center diode 13. Arrangedbetween the anode of each diode and ground is a by-pass capacitor 34.The by-pass capacitors prevent radio frequency leakage from theattenuator through the DC. power supply leads. They also provide a lowimpedance radio frequency return at the diode anodes, and they decouplethe diode sections from one another thus increasing the maximum value ofattenuation attainable.

The operation of the attenuator is as follows:

If minimum attenuation of the power level between input terminal andoutput terminal 11 is desired, i.e., if maximum power is to appear atterminal 11, the potentiometer 22 is adjusted to introduce its largestpossible resistance into the circuit. As a result, there is almost noDC. voltage drop'across the diodes 12-14 and consequently, the diodesact substantially as open circuits, i.e., they offer extremely highresistance to the flow of A.C. current from the source connected toterminal 10. This is due to the well known characteristic curve of adiode which indicates that at low voltages, the current through a dioderemains substantially constant throughout variation of voltage. SinceAV=AI-R, where AV is change in voltage drop across the diode, AI iscurrent flowing through the diode, and R is the resistance offered bythe diode, it is clear that if AV is large but AI remains very small, Rmust be very large.

Current from the A.C. source, entering the attenuator through terminal10 divides when it reaches the junction '29; a portion of the currentflows through padding resistor 30 and the remainder flows through thefirst isolating resistor 25. Upon reaching the junction 15, the currentflowing through resistor 25 tends to divide so that a portion flowstoward diode 12 and the remainder flows through resistor 23. However,under the circumstances mentioned above, the resistance of diode 12 isso large that substantially all of the current flows through resistor23. Similarly, at junctions 16 and 17, substantially no current flowstoward the diodes 13 and 14, respectively. Hence, nearly all the currentwhich flows through resistor 25 reaches terminal 31. At this point thecurrent divides between padding resistor 32 and the load connected toterminal 11.

If maximum attenuation is desired, i.e., minimum power at terminal 11,the potentiometer 22 is adjusted to introduce minimum resistance intothe circuit, whereby a high current flows to the diodes 1214. As aresult, the resistance of the diodes is very small and they actpractically as open circuits. This is again due to the diodecharacteristic which indicates that at high currents, the voltage dropacross a diode remains substantially constant. Thus, in the expressiongiven above, if AV approaches zero, and AI is large, R must approachzero.

When the portion of the current from terminal 10, which flows throughthe resistor 25, reaches the junction 15, under the circumstances justdescribed almost all of the current will flow toward the diode 12, sinceits re sistance is very small. The small portion of the current flowingthrough the resistor 23 is again divided at junction 16 in such a waythat most of it flows toward diode 13. It will be appreciated,therefore, that almost no A.C. current reaches the terminal 11.

Obviously, if the potentiometer 22 is adjusted to an intermediateresistance value, the current reaching each junction -17 will divide inproportion to the resistance value of the diode connected to thatjunction and the resistor 23, 24, or 26 immediately following thejunction. Thus, since the potentiometer 22 permits continuous variationof the current flowing to the diodes and hence the resistance of thediodes, the arrangement as a whole permits continuous variation of theattenuation produced by the attenuator.

In order to clearly describe the function of the padding resistors 30and 32, it will be helpful to assign illustrative values to theresistors 23-26, 30, and 32. It is to be understood, however, that thesevalues are presented for the sake of explanation and are not intended tolimit this disclosure in any way. Assume that each resistor 23-26 has avalue of 220 ohms, and that resistors 30 and 32 each has a value of 56ohms. Assume also that the attenuator is designed to have acharacteristic impedance of 50 ohms.

If resistors 30 and 32 were not present, the input impedance of theattenuator under the conditions of minimum attenuation would beessentially the sum of the resistance values of the resistors 23-26,since almost all of the current from terminal 10 would flow through allthe resistors. On the other hand, under conditions of maximumattenuation, the input impedance of the attenuator would be essentiallythe value of the first resistor 25, since almost no current flowsthrough the other resistors. Clearly, therefore, it would be impossiblefor the attenuator to have an input impedance, and for similar reasonsan output impedance, which remains close to the characteristic impedanceof the attenuator at both maximum and minimum attenuation. This isundesirable because under these circumstances, the input and outputvoltage standing wave ratio (VSWR) will not be close to unity. VSWR maybe defined as the ratio of the maximum to minimum voltage existing in anarbitrarily long lossless transmission line of characteristic impedanceZ connected to a load of km pedance Z.

Provision of the padding resistors 30 and 32 reduces the input andoutput VSWR of the attenuator to a value close to one. Using the valuegiven above, it will be seen that at minimum attenuation, the minimumvalue of input impedance is 52.6 ohms. This value is calculated asfollows:

=52.6 ohms impedance)=1.056. At maximum attenuation, the minimum valueof input impedance is 48.3 ohms. This value is calculated as follows:

220(56) 220+56 The input VSWR is therefore: 50/48.3=1.035. Similarcalculations can be made to determine the output VSWR of the attenuator,and these would be identical to the input VSWR values. It will be seen,therefore, that the present invention provides an attenuator having aninput and an output VSWR very close to unity, and certainly far lowerthan the VSWR of 1.5 to 2 which can ordinarily be tolerated incomponents of this type.

The invention has been shown and described in preferred form only and byway of example, and many variations may be made in the embodimentdescribed which will still be comprised within the spirit of theinvention. It is understood, therefore, that the invention is notlimited to any specific form or embodiment except insofar as suchlimitations appear in the appended claims.

What is claimed is: I

1. An attenuator adapted to be connected between a source of alternatingcurrent power and a load for the purpose of adjusting the amount ofpower supplied to the load, comprising an electriccircuit between thesource and load, said circuit including a plurality of series connectedjunctions, plural voltage variable resistancemeans each having a firstand a second terminal and each having said first terminal connected toone of said junctions,

min

=48.3 ohms means for supplying a selected DC. voltage to said secondterminal of said variable resistance means said means for supplying aselected DC. voltage having a point of ground reference potential incommon with said source of alternating current power and said load, anisolating resistor between each of said junctions and between theendmost junctions and said source and load, and padding resistorsconnected from junctions between the endmost of said isolating resistorsand said A.C. source and load to said point of ground potential.

2. An attenuator as defined in claim 1 where each of said variableresistance means is a diode.

3. An attenuator as described in claim 2 including at least three ofsaid diodes, the anodes of said diodes being connected to a commonjunction, and a balance resistor between each of said endmost diodes andsaid common junction.

6 References Cited UNITED STATES PATENTS OTHER REFERENCES Johnson:Transmission Lines and Networks, McGraw- Hill, New York, copyright 1950,pages 286289 relied on.

15 HERMAN KARL SAALBAOH, Primary Examiner.

R. F. HUNT, Assistant Examiner.

1. AN ATTENUATOR ADAPTED TO BE CONNECTED BETWEEN A SOURCE OF ALTERNATINGCURRENT POWER AND A LOAD FOR THE PURPOSE OF ADJUSTING THE AMOUNT OFPOWER SUPPLIED TO THE LOAD, COMPRISING AN ELECTRIC CIRCUIT BETWEEN THESOURCE AND LOAD, SAID CIRCUIT INCLUDING A PLURALITY OF SERIES CONNECTEDJUNCTIONS, PLURAL VOLTAGE VARIABLE RESISTANCE MEANS EACH HAVING A FIRSTAND A SECOND TERMINAL AND EACH HAVING SAID TERMINAL CONNECTED TO ONE OFSAID JUNCTIONS, MEANS FOR SUPPLYING A SELECTED D.C. VOLTAGE TO SAIDSECOND TERMINAL OF SAID VARIABLE RESISTANCE MEANS SAID MEANS FORSUPPLYING A SELECTED D.C. VOLTAGE HAVING A POINT OF GROUND REFERENCEPOTENTIAL IN COMMON WITH SAID SOURCE OF ALTERNATING CURRENT POWER ANDSAID LOAD, AN ISOLATING RESISTOR BETWEEN EACH OF SAID JUNCTIONS ANDBETWEEN THE ENDMOST JUNCTIONS AND SAID SOURCE AND LOAD, AND PADDINGRESISTORS CONNECTED FROM JUNCTIONS BETWEEN THE ENDMOST OF SAID ISOLATINGRESISTORS AND SAID A.C. SOURCE AND LOAD TO SAID POINT OF GROUNDPOTENTIAL.